In a magnetic disk apparatus,in order to perform recording and reproduction on a target track of an off-centered magnetic disk with a magnetic head, a magnetic head control apparatus is necessary for causing the magnetic head to follow the target track which varies continually in distance from a revolution center owing to the eccentricity. A conventional eccentricity control apparatus of a prior art magnetic head control apparatus comprises a sine wave and cosine wave generator, a digital Fourier transformer and a multiplier. The sine wave and cosine wave generator outputs sine wave and cosine wave signals corresponding to the revolution frequency of a motor of a magnetic disk apparatus. The digital Fourier transformer performs digital Fourier transformation on the position error relative to a target position of the magnetic head on the basis of the above-mentioned sine wave and the cosine wave signals. The position error processed by the digital Fourier transformation is multiplied by the weight coefficient of each of the sine wave and cosine wave vectors, and then a signal of the value obtained by adding both is made an eccentricity correction signal. The eccentricity correction signal is used for the control of the magnetic head so as to compensate the eccentricity of the magnetic disk and thereby cause the magnetic head to follow the target track. The result of the above-mentioned digital Fourier transformation is multiplied and summed (hereinafter is referred to as multiplication and summation operation) repeatedly for each revolution of the magnetic disk. Then, the position error which has been reduced by the first compensation is processed by the multiplication and summation operation into the second digital Fourier transformation result. The second compensation realizes a state equivalent to a further reduction of eccentricity, and hence the position error is reduced further. As such, by performing the multiplication and summation operation of the digital Fourier transformation result, the position error due to eccentricity can be theoretically reduced into infinitesimal.
In the above-mentioned magnetic head eccentricity control apparatus, a signal indicating the position error from the position (hereinafter is referred to as a target position) of the target track on which the magnetic head is to follow is processed by digital Fourier transformation on the basis of the sine wave and cosine wave signals. Each of the sine wave and cosine wave vectors of the signal processed by the digital Fourier transformation is multiplied by a weight coefficient, and then the value of the sum of both is outputted as an eccentricity correction signal. This results in a large amount of the multiplication and summation operation, and hence needs a large amount of time in the calculation. The control delay due to the necessity of a large amount of time in the calculation causes directly an adverse effect to the control performance. With increasing track density in the future, the sampling frequency in the digital Fourier transformation will become higher, whereby the speed-up of the calculation in the control apparatus is required in order to meet the higher frequency. Further, during the seek and the settling of the magnetic head, a large variation occurs in the position error of the magnetic head. Consequently, there is a case that the eccentricity correction signal is not correctly calculated, and the control performance is prone to deteriorate.